1. Field of the Invention
This invention relates to a method of recording an X-ray image, and more particularly to a method of recording an X-ray image by use of a stimulable phosphor sheet.
2. Description of the Prior Art
For medical diagnosis, there is generally used radiography in which a radiation image of an object is obtained by exposing the object to X-rays and recording the X-rays transmitting through the object on an X-ray photographic film. In conventional radiography, since a radiation image of the whole object is recorded, the obtained radiograph is sometimes adversely affected by an interfering shadow overlapping upon the image of a portion of the object under examination. For example, when the lungs are radiographed, a shadow of the rib overlaps upon the image of the lungs recorded, making it difficult to correctly diagnose the area of disease in the lungs. When abdomenal organs are radiographed, gas contained in the intestines forms a shadow overlapping on the image of the abdomenal organs and adversely affecting the diagnosis of the organs.
To eliminate the interfering shadows encountered in conventional radiography, it has been proposed to conduct tomography in which an image of only a desired tomographic plane in an object is sharply formed on an X-ray photographic film while images of the other planes in the object are made unsharp. In tomography, an X-ray tube and an X-ray photographic film are opposed to each other with an object therebetween, and moved with respect to each other around the object at the time of exposing the object to X-rays so as to satisfy the linear rule and the geometric rule. The linear rule specifies that the focal point of the X-ray tube, one point on the tomographic plane in the object, and one point on the X-ray film must be on the same straight line. The geometric rule specifies that the ratio of the distance between the focal point of the X-ray tube and the tomographic plane in the object to the distance between the tomographic plane and the X-ray film must be maintained constant.
Recently, it has been proposed to conduct simultaneous multilayer tomography in which recording is effected in the same way as described above by stacking a plurality of combinations of an X-ray photographic film and two X-ray intensifying screens to simultaneously obtain images of a plurality of tomographic planes in an object on the respective X-ray photographic films of said combinations.
In tomography, the X-ray tube and the combinations may be moved along linear, circular, elliptical or spiral paths insofar as the linear rule and the geometric rule described above are satisfied. Tomographic techniques are described in detail, for example, in "Hoshasen Gijutsu No Tebiki" (Guide to Radiation Technology) published by the Tokyo Radiation Engineers' Association.
FIG. 1 schematically shows conventional simultaneous multilayer tomography in which an X-ray tube and combinations of an X-ray photographic film and X-ray intensifying screens are moved along horizontal linear paths. An X-ray tube 1 and a cassette 3 containing a stack of X-ray photographic films 2a, 2b and 2c are opposed to each other on both sides of an object 4. The X-ray photographic films 2a, 2b and 2c are provided with X-ray intensifying screens (not shown) so that a lower film exhibits a higher sensitivity to X-rays than an upper film.
In the recording step, the object 4 is exposed to X-rays emitted from the X-ray tube 1 while the X-ray tube 1 and the cassette 3 are moved in the directions of the arrows 5 and 6, respectively. In this way, an image of a tomographic plane 8a which is defined by the intersections of X-rays 7a and 7a', respectively, formed as the X-ray tube 1 is moved in the direction of the arrow 5 is formed on the X-ray photographic film 2a. Similarly, an image of a tomographic plane 8c which is defined by the intersections of X-rays 7c and 7c', respectively, formed as the X-ray tube 1 is moved is formed on the X-ray photographic film 2c.
In simultaneous multilayer tomography, images of a plurality of tomographic planes in an object can be recorded on X-ray photographic films by a single recording step. Accordingly, for moving organs such as the heart and lungs, it is possible to obtain tomographic images in exactly the same motion phase. For stationary organs, it is possible to obtain images of a plurality of tomographic planes, which are free from adverse effects due to movement of the object in the course of tomography. Furthermore, simultaneous multilayer tomography reduces the X-ray dose to the object (patient) and reduces the physical and mental burden on the patient.
However, in conventional simultaneous multilayer tomography, X-rays transmitting through the object are attenuated as they sequentially transmit through a plurality of combinations of an X-ray photographic film and X-ray intensifying screens, so that an upper X-ray photographic film exhibits a higher image density and a lower one a lower image density. Therefore, it is necessary to compensate for the fluctuation in image density among the X-ray photographic films by adjusting the sensitivities of the combinations of the X-ray photographic film and the intensifying screens. In general, however, it is very difficult to adjust the sensitivities thereof according to respective objects. Further, the sensitivity of the lowermost combination can be increased only to a limited extent, i.e. only to an insufficient level. In addition, X-rays of lower energy are absorbed in an upper combination and do not reach a lower one, and X-rays of higher energy travel a longer distance to a lower combination. Accordingly, an upper X-ray photographic film exhibits a higher contrast and a lower one a lower contrast, necessitating adjustment of the gamma values of the X-ray photographic films. However, it is very difficult to prepare X-ray photographic films with gamma values adjusted according to the respective objects. For the reasons mentioned above, it is generally extremely difficult to obtain X-ray images which can be used for diagnosis with high efficiency and accuracy with conventional simultaneous multilayer tomography. Further, in conventional simultaneous multilayer tomography, at most only four or five layers of the combinations of the X-ray photographic film and intensifying screens can be stacked and used for tomography. Therefore, to obtain more tomographic images, it is necessary to conduct the recording step many times.
As a method of obtaining an X-ray image of an arbitrary slice thickness of an object, it has been proposed to change the distances of the movements of the X-ray tube and the combinations conducted with respect to each other around a tomographic plane of the object so as to satisfy the linear rule and the geometric rule described above. In this method, images of a small slice thickness of the object are obtained when the distances of the movements of the X-ray tube and the combinations with respect to each other are increased, and images of a large slice thickness of the object are obtained when the distances of the movements thereof are small. However, when the X-ray tube and the combinations are moved a short distance with respect to each other to obtain X-ray images of a large slice thickness, e.g. 5 mm or more, shadows detrimental to diagnosis are not eliminated completely from the X-ray images, and the effects of tomography cannot be achieved. Thus, with this method, it is extremely difficult to obtain X-ray images of an arbitrary slice thickness, particularly 5 mm or more, which are free from the shadows interfering with diagnosis.